Various kinds of seat belt apparatus having a pre-tensioning function are already known, in which a seat belt is retracted to strongly restrain a vehicle occupant sitting in a vehicle seat, when a possible crash is predicted. The seat belt is generally fastened in an unstrained condition, because the seat belt may give an unpleasant feeling to the vehicle occupant when it is tightly fastened. The seat belt, however, as the case may be, can not assure safety for the vehicle occupant depending on largeness of the crash, when the vehicle crashes in a condition that the seat belt is fastened in the unstrained condition.
According to a seat belt apparatus, for example, as disclosed in Japanese patent No. 2 946 995 or No. 2 868 952, the safety for the vehicle occupant is more reliably assured at a vehicle crash, by retracting the seat belt in advance to eliminate its loose when a possible vehicle crash is predicted.
A seat belt detecting device, such as a buckle switch, is generally provided in the seat belt apparatus for detecting whether the seat belt is fastened or not, so that the seat belt apparatus can determine based on a detected signal from the seat belt detecting device that the seat belt is fastened. And the seat belt is retracted when the vehicle crash is predicted. As a device or means for predicting the possible vehicle crash, a vehicle control device such as an acceleration sensor, a detecting device for a vehicle stopping lamp, a detecting device of a vehicle brake, or a vehicle distance detecting device such as a millimeter-wave radar device, a laser radar device or the like, can be used.
When the acceleration sensor (G sensor) is used as the crash predicting means, the possible vehicle crash is predicted based on a detection of a strong brake operation. In the case that the detecting device for the vehicle stopping lamp is used as the crash predicting means, the vehicle crash is predicted based on the detection of the operation of the lamp. In the case that the detecting device of the vehicle brake is used as the crash predicting means, the vehicle crash is predicted based on a pressure change of braking fluid at a master cylinder of a braking device. Furthermore, when the millimeter-wave radar device or the laser radar device is used as the crash predicting means, the vehicle crash is predicted based on the detected distance between the self vehicle and an obstacle or another vehicle running on an opposite lane.
The above crash predicting means may erroneously output a crash predicting signal, even when there is no danger for an actual vehicle crash, because the possible vehicle crash is predicted based on a change of vehicle running condition in any of the above crash predicting means. The seat belt may be retracted even in the case that there is no danger of the crash, and such operation may give an unpleasant feeling to the vehicle occupant.
Further, in the case that the child is sitting in the seat, the unpleasant feeling to the child would become larger, when the seat belt is retracted.